Catalytic cracking is a petroleum refining process that is applied commercially on a very large scale. A majority of the refinery petroleum products are produced using the fluid catalytic cracking (FCC) process. An FCC process typically involves the cracking of heavy hydrocarbon feedstocks to lighter products by contacting the feedstock in a cyclic catalyst recirculation cracking process with a circulating fluidizable catalytic cracking catalyst inventory consisting of particles having a mean particle size ranging from about 50 to about 150 μm, preferably from about 50 to about 100 μm.
The catalytic cracking occurs when relatively high molecular weight hydrocarbon feedstocks are converted into lighter products by reactions taking place at elevated temperature in the presence of a catalyst, with the majority of the conversion or cracking occurring in the vapor phase. The feedstock is converted into gasoline, distillate and other liquid cracking products as well as lighter gaseous vaporous cracking products of four or less carbon atoms per molecule. The vapor partly consists of olefins and partly of saturated hydrocarbons. The vapor also contains bottoms with coke being deposited on the catalyst. It is desirable to produce the lowest bottoms at a constant coke level.
FCC catalysts normally consist of a range of extremely small spherical particles. Commercial grades normally have average particle sizes ranging from about 50 to 150 μm, preferably from about 50 to about 100 μm. The cracking catalysts are comprised of a number of components, each of which is designed to enhance the overall performance of the catalyst. Some of the components influence activity and selectivity while others affect the integrity and retention properties of the catalyst particles. FCC catalysts are generally composed of zeolite, active matrix, clay and binder with all of the components incorporated into a single particle or are comprised of blends of individual particles having different functions.
Bottoms upgrading capability is an important characteristic of an FCC catalyst. Improved bottoms conversion can significantly improve the economics of an FCC process by converting more of the undesired heavy products into more desirable products such as light cycle oil, gasoline and olefins. Conventional wisdom has suggested that as matrix surface area increases in a cracking catalyst, the yield of bottoms decreases.
One attempt to increase catalytic matrix surface area has been to control alumina distribution within an alumina sol based cracking catalyst. For example, aluminum sol based FCC catalysts typically consist of a zeolite (e.g. faujasite zeolite), one or more matrix aluminas and/or silica-aluminas, clay (e.g. kaolin clay) bound with an aluminum chlorhydrol binder. The catalysts are typically prepared by spray drying an aqueous slurry of the zeolite, clay and alumina chlorhydrol. The sprayed dried catalyst particles are thereafter calcined typically at a temperature of about 595° C. and, optionally, ion exchanged to remove undesirable impurities.
To increase matrix surface area, however, the sprayed catalyst particles have been calcined at a milder calcination temperature, e.g. about 400° C. The calcined catalyst particles were then re-slurried with an aqueous based solution at a pH of about 7 to re-dissolve alumina from the binder system and re-precipitate alumina on the surface of the zeolite containing catalyst particles. While this process can generate an increase in matrix surface area on the surface of the final catalyst, the process has undesirable limitations. A major limitation of this process has been the proportional relationship between the increase in matrix surface area generated and the amount of zeolite in the catalyst system. That is, an increase in the matrix surface area requires a proportional increase in the amount of zeolite in the catalyst system. An increase in zeolite content, however, may not be desirable when attempting to maximize LCO yield and can thereby limit the range of application of the catalyst.
In addition to increased bottoms conversion, it is also important to avoid or minimize the output of coke during an FCC process. Consequently, there exists a need in the refining industry to provide catalysts that minimize coke formation while simultaneously enhancing bottoms cracking performance during a catalytic cracking process.